Water-cooled nuclear reactors comprise a vessel containing the core of the reactor and connected to the reactor cooling circuit in which the cooling water circulates.
The internal equipment, consisting of various components fastened the vessel, makes it possible in to ensure that the core assemblies are supported and retained and that the cooling water is channelled within the vessel.
This internal equipment comprises in particular a core shroud consisting of a shell fastened coaxially with the vessel in the form of a generally cylindrical body closed by domed bottoms and arranged with its axis in the vertical direction.
The internal equipment of the reactor, which equipment is in contact with the cooling fluid and exposed to the radiation emitted by the reactor core, is highly activated and contaminated after the reactor has been in operation for some time.
As regards power stations which have reached the end of their lives and which require a complete shutdown, the past solution has been to leave these power stations in their existing state and to allow the activity of the constituent materials of their components to decrease, for the purpose of subsequently dismounting them under conditions more satisfactory than those at the time of the shutdown, without the need to employ complex remote-controlled tools.
Since the number of power stations put out of industrial operation is expected increase appreciably, it is necessary to consider dismantling these power stations so that the site where they are built can be restored to its original state.
While dismantling of the conventional part of the power station presents no particular problem, the dismantling of the part of the power station constituting the actual nuclear reactor, and particularly the internal equipment of the vessel, poses problems difficult to solve in view of the radioactive emissions of the constituent materials of the reactor components.
The reactor vessel is arranged inside a vessel well formed in a concrete structure which also delimits one or more pools arranged above the upper level of the vessel.
After permanent shutdown and cooling of the reactor, the pool is filled with water and the vessel cover is removed.
The underwater unloading of the core assemblies and the disposal of these assemblies, for example to reprocessing factories, are then carried out.
It is then necessary to dismantle and dispose of the internal equipment of the vessel, and these operations have to be conducted under a large depth of water because the internal equipment is highly irradiated.
The dismantling of the internal equipment must therefore be controlled remotely from the upper edge of the reactor pools, after these pools have been filled to their maximum level.
The various components of the internal equipment are dismounted and/or cut underwater inside the vessel, so as to obtain elements which are separated from the vessel and the disposal of which is carried out inside casks or containers.
It is possible, as a result of mechanical, thermal or electrochemical cutting of the components of the internal equipment inside the vessel, to obtain fragments which are sufficiently small to be introduced into the casks or containers and to be stored in these without a subsequent volume-reducing operation. However, such cutting techniques give rise to the formation of a volume of radioactive waste which is proportional to the cutting lengths. Conforming the separate elements of the internal equipment to the sizes of the casks or containers by cutting is therefore liable to produce a large volume of waste of very high activity, the recovery and processing of which may prove very difficult.
It is consequently preferable to employ techniques for bringing the fragments of components of the internal equipment extracted from the vessel to an appropriate size which can be put into practice without the removal of material.
Hot or cold cracking of the components or elements of the internal equipment has been considered, but this technique seems to be of limited use in view of the thicknesses of the parts or walls to be broken down by cracking and the adaptation of the process to the configuration of the fragmented parts. In fact, it is necessary to carry out an adjustment of the parameters of the cracking operation as a function of the configurations of each of the components of the internal equipment.
It has thus not been no known how to dismantle the internal equipment of a water-cooled nuclear reactor, allowing components or elements of the internal equipment to be brought to an appropriate size, by a method which is simple and which produces minimal radio active waste difficult to recover.